1. Field of the Invention:
This invention relates to a still-video camera (an electronic still camera) capable of frame recording (frame photography) using a shutter.
The term "frame recording" is used in contradistinction to "field recording". In field recording, one frame (one image) of an image is expressed using one field of a video signal, and one field of a still-video signal is recorded on a recording medium in order to record one frame of an image. In frame recording, on the other hand, one frame of an image is expressed using first and second fields, for a total of two fields (one frame) of a still-video signal in interlaced scanning, and a still-video signal of two fields per frame image is recorded on the recording medium.
2. Description of the Related Art:
In a still-video camera, it is possible to control the storage time of an electric charge in a solid-state electronic imaging device by using a shutter. Since storage of the electric charge in the imaging device is performed during the time that the shutter is open (i.e., during the exposure time), read-out from the image pick-up device is prohibited at this time. When the shutter is closed (i.e., when the exposure ends), the charge is read out of the imaging device at a predetermined timing and the read still-video signal is recorded on a recording medium (which may include a magnetic disk and a semiconductor memory).
In an imaging device, a dark current is produced at all times. As a result of the dark current, charge continues to be stored during the passage of time regardless of whether the imaging device is being exposed or not. The level of the dark current varies in accordance with the charge storage time.
In the frame recording mode of operation for a still-video camera, read-out of the video signal (stored charge) from the imaging device is performed by interlace scanning, so that a still-video signal of two fields is read-out one field at a time. However, there is generally a difference of one vertical scanning interval between the storage time of dark current in the first field and the storage time of dark current in the second field. As a result, a difference develops between the levels of these dark currents. Therefore a considerable difference exists between the brightness levels of the two fields of video signals forming one frame. When a video signal containing such a dark current is recorded on a recording medium, there is the danger that flicker will be produced when the signal is played back for display on a display unit such as a CRT, which will result in an unattractive picture.
This operation will now be described in greater detail with reference to FIG. 7. Read-out from a solid-state electronic imaging device is performed by reading out a first field (field A) in response to a field-shift signal FSA and a second field (field B) in response to a field-shift signal FSB. These field-shift signals FSA and FSB are generated alternately at a fixed period. A field-shift enable signal FSEN controls whether readout from the imaging device in response to the field-shift signals FSA and FSB is to be enabled or disabled. The signal FSEN attains an H level to disable read-out during the time that the shutter is open (i.e., during the time that a shutter-closed signal S.sub.close is at the H level). In the example of FIG. 7, only read-out in field B is disabled (the disabled signal FSB is indicated by the dashed line b), so that a dark current i.sub.DARKB in field B continues to increase without being swept out. Accordingly, in read-out and recording (the interval during which the signal REC is at the H level) performed after the shutter is closed, a problem that arises is that the level of the dark current i.sub.DARKB contained in the read signal of field B becomes higher than the level of the dark current i.sub.DARKA in the read signal of field A.
It is known that noise due to smear is superimposed on the video signal when a CCD image sensor is used as the imaging device. Noise attributable to smear is caused in such a way that specifically, when light impinges upon the light-receiving section, light, particularly in the infra-red region, induces unnecessary electric charge in portions other than the light-receiving section, such as in the substrate portion of the CCD image sensor, and the charge so produced becomes mixed in the transfer path.
In order to eliminate smear-induced noise, an effective method entails clearing the transfer path at the completion of exposure to remove excess electric charge from the transfer path, followed by transmitting the charge stored up in the light-receiving section to the transfer path (i.e., reading out the signal).